With an average of one hit per year, is it possible to separate your lightning protection from your safety grounding for the building?
Assuming you're in for 'overkill', provide separate protection by installing tower(s) with lightning rod(s) around your building taller than your building. Rods will 'leak' charge into the clouds; provide 'first hit' points providing separate discharge paths generally routing the energy away; *and*, best of all, form an umbrella shade for high voltage fields. How many? How far away? How tall? I recommend placing towers a distance away from your protected structure of at least 1.5 times the height [away >from the building edges] and calculate the height of the towers by assuming a cosine shaped cone down from the tip. See finite element analyses mentioned later. Make sure your building is inside all cones. One tower tall enough will suffice, but 4 probably are easier to install and have less visual pollution. When lightning hits, it will be 'softer' due to the pointy, but still there for a bit. Count on a LOT of voltage at that tip. Check everywhere that there are no sharp edges towards the rod and its ground wire. Else, you have a potential of a secondary arc over between the rod structures and your building that you do not want. Next, assume you'll get a 10,000 amp spike down the return path(s) and check to make sure you have NO parallel wires to pick that field up, else you'll induce a transformer like action and shove floating voltages into your wiring - you don't want that either. You can calculate/estimate the transformer coupling by using an approximation of 100MHz, 10K Amps. If you have access to 3D finite element analyses, best, if not, free femm 4.2 works great. femm will also do perfect 3d axisymmetric views of an electrostatic field to help decide distances. Next you should consider the 'voltage gradient' effect of a discharge through the return path. The earth is not that great a conductor. So when currents flow a voltage gradient will appear. To invision these fields, picture the earth as a giant rubber sheet representing ground plane, then when ESD hits a spot on it, it is like pinching the rubber sheet at that spot and pulling it up to high voltage. Now you have a kind of view of the voltage gradients out away from that point. It's obvious that a wire connecting two points strung radially will be catastrophically connecting two points with a high voltage difference, but connecting axial will be ok. In other words, avoid connecting any two points in that plane with hard wires *if* there will be a large voltage differential, else secondary arcing CAN/WILL occur. This is why a four legged animal is more likely to be killed than a two legged animal. The voltage gradients along the dirt ground can be large. As a last over kill put all your wiring ingress/egress on double high voltage relay lines to enable you to 'cut' power and telephone lines to your building during really dangerous conditions. Picture outside line, first breaker set, second breaker set, and building wiring. Between the first and second breaker contacts, place arc-over tranzorbs and gas tubes to form paths to your ground structure and you should catch the last remnants of lightning energy. Extra protection: any wiring that compromises what you're trying to do, place common mode ferrite beads/clamps every six inches for distances of 10 to 20 feet, no loop backs. Remember the rubber sheet allegory? That allows you to maintain copper connections but still maintain some degree of spike protection. Discharge spectrum usually centers between at 1, 3, to 30 MHz, but the energy tails go way out [both low and high] passed 100-200MHz. You can quantitatively check a lot of your grounding and wiring techniques by using a spectrum analyzer with a tracking generator option. Robert > Morning all, > > I'm pondering which path to follow with a grounding issue. I'm not so > much concerned with code requirements, I see these as the minimum, I'm > after best... > > First some background. I have built a building to house my business. > I've located this well in the boonies on some 23 acres of woods. In > order to blend in it's in the shape of a classic Midwest USA barn. It > was custom engineered from Steel red-iron and Steel siding. > > My quandary is how best to ground this for electrical safety and > lightning. It's a fairly large building ( 50 feet by 80 feet ) and > reaches almost to the tops of surrounding trees. Terrain wise we seem to > have a fair number of strikes, in the last 5 years we have lost 3 trees > to lightning. > > If I follow guidance I have been given to ground the building only where > the power enters, I see this as a week point during a lightning event. > > A bit more about the building. If you can imagine 5 huge horseshoes > stood upright on the open end, thats what the main frame looks like. > There is of course a large amount of interconnecting red iron and two > additional vertical columns at each end of the building. Over the whole > frame is steel siding. The whole thing is bolted together with many 1 > inch and 1/2 inch bolts. The siding has close to 8000 screws fastening > it to red iron. > > The building stands right now about 1 foot above "dirt" on a concrete > footing wall, and there is a 10 foot wide concrete path to be installed > all around the building making up the 1 foot height difference: the > solution to my quandary is needed before the concrete can be installed. > Soon would be nice so I can quit getting muddy boots :-) > > I'm leaning towards installing a copper grounding ring all around the > building say 4 to 5 feet or maybe up to 10 feet away from the footing > wall. This ring will be about a foot down in the dirt. At the point > where each building column touches the concrete I'm looking to install a > heavy ( 4/0 ) grounding wire out to the wire ring. At the point they > meet I'll drive a 10 foot grounding rod flush with the wire ring and > join them with exothermic welds. 4/0 wire is easy to get here, but > copper strips may also be an option. > > I'm looking for thoughts, opinion, recommendations on whether this > approach is sound, sufficient, overkill, under kill etc. Both I, and my > boots, would appreciate the groups thoughts. > > Sincerely, > > Derek Walton > L F Research > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to that > URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
